I recently had the IT guys here at work wipe my hard drive and upgrade my mac to 10.8 from 10.6. And other than spending a few days getting it up and running again and spaces being jacked it is going ok.

I was using Time Machine to keep it backed up for a long time and I want to use it again on the same external drive. But I am concerned that my most recent backup from my old backups will be lost if I set it up using the same drive. Will Time Machine wipe all my old data or just the oldest stuff?

Since I don't have a Mac handy to test this scenario on, I can't definitively answer. But I know it won't delete your prior Mac's backup. It will instead either upgrade and continue on your existing backup or (more likely) it will ignore that backup and start a new one alongside it. Either way it will not delete files already there without your prompting it to do so.
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bispymusicMar 11 '13 at 21:37

2 Answers
2

No.

Time Machine will just start backing up your Mac along side your old Backups. Your previous machine will be back on Tuesday (or whenever last backed-up) in your timeline, with the new machine after that.

I have just bought and setup a new machine ubde =r Mountain Lion (and using a remoter Time Machine)

I created it from the Time Machine backup of the old one. Time machine then gave me the option of 'hijacking' the old backups and continuing to add to them which has worked, I now have history of my backups from now back to before I created the new machine. (My old machine had a bad disk and is very old so I don't use it anymore)